Pressure driven separation membrane, such as reverse osmosis (RO), ultrafiltration (UF) and nanofiltration (NF), are of important roles in many industrial applications where the organic molecules or inorganic ions in aqueous solutions can be concentrated or separated by a positive osmotic pressure to one side of a filtration membrane.
Therein nanofiltration (NF) is a membrane filtration capability which can be used to filter both natural and synthetic organic compounds. Nanofiltration is relatively a newer filtration strategy ranging in between ultrafiltration (UF) and reverse osmosis (RO). The nominal pore size of NF membrane is typically around 1 nm For application purposes, the rejection capability of NF membranes is generally evaluated by molecular weight cut-off (MWCO) instead of the nominal pore size. The molecular weight cut-off for NF is normally regarded as 400 Da. The required transmembrane pressure (pressure drop across the membrane) for NF is at least 0.5 MPa, but this pressure is much lower than that used for RO. Therefore, the capital cost (the need for system to withstand high pressure) and operation cost (the energy input to generate high pressure) for NF can be significantly reduced.
The NF membranes can be designed with a composite structure and fabricated by performing the interfacial polymerization reaction onto a microporous polymer support. Nanofiltration membranes are structurally very similar to reverse osmosis membranes and typically are cross-linked aromatic polyamides cast as a thin layer. Nowadays, nanofiltration membranes are widely applied in the industrial wastewater treatment, power generation, food & beverage processing, municipal desalination and water reuse, and home drinking water devices, etc. Currently, several companies, such as GE, Koch, TriSep, Dow FilmTec, are recognized as the leaders in RO and NF commercial membrane products.
U.S. Pat. No. 5,152,901 provides a method to prepare polyamine-polyamide composite nanofiltration membrane for water softening.
U.S. Pat. No. 5,922,203 discloses amine monomers and their use in preparing interfacially synthesized membranes for reverse osmosis and nanofiltration.
U.S. Pat. No. 6,464,873 discloses an interfacially polymerized process for reverse osmosis on a microporous support with an essentially monomeric bipiperidine reactant, and an essentially monomeric amine-reactive polyfunctional aromatic or cycloaliphatic acyl halide.
U.S. Pat. No. 6,833,073 provides a method for producing nanofiltration and reverse osmosis membranes that may be used in a number of commercial applications.
U.S. Pat. No. 7,001,518 discloses the production method for a low pressure reverse osmosis and nanofiltration membranes.
U.S. Pat. No. 7,138,058 discloses an acid stable membrane for nanofiltration, which allow for the efficient processing of feed solutions containing acids.
However, there are still needs to further create new products with improved performance, cost-effective fabrication, and diversified configurations to fit the increasing demands of wider NF application. Current NF patents and products are mostly for sheet membranes, but the hollow fiber membranes have increasingly adopted for many filtration needs due to its easy cleaning and lower fouling tendency.
Consequently, there is an unmet need to have an efficient and cost-effective method to fabricate hollow fiber NF membrane composite products aiming for separating or concentrating organic compounds from solutions with a lower operation pressure and higher rejection rate.